Varying switching frequency and period of a power supply controller

ABSTRACT

An example integrated circuit controller for use in a switching power supply includes a pulse width modulation (PWM) circuit and a timing circuit. The PWM circuit controls a switch to regulate an output of the power supply in response to a switch current flowing through the switch and in response to a clock signal having a switching period. The timing circuit provides the clock signal and increases the switching period in response to an on time of the switch exceeding a threshold time.

CROSS-REFERENCE TO RELATED APPLICATION

This application is related to co-pending U.S. application Ser. No. 13/193,434, entitled “Variable Frequency Timing Circuit for a Power Supply Control Circuit,” filed on the same day herewith.

BACKGROUND INFORMATION

1. Field of the Invention

The present invention relates generally to power supplies, and more specifically, the present invention relates to controllers for switched mode power supplies.

2. Background

Electronic devices use power to operate. Switched mode power supplies are commonly used due to their high efficiency, small size and low weight to power many of today's electronics. Conventional wall sockets provide a high voltage alternating current. In a switching power supply a high voltage alternating current (ac) input is converted to provide a well regulated direct current (dc) output through an energy transfer element. In operation, a switch is utilized to provide the desired output by varying the duty cycle (typically the ratio of the on-time of the switch to the total switching period), varying the switching frequency or varying the number of pulses per unit time of the switch in a switched mode power supply.

The switched mode power supply also includes a controller which usually provides output regulation by sensing and controlling the output in a closed loop. The controller may receive a feedback signal representative of the output and the controller varies one or more parameters in response to the feedback signal to regulate the output to a desired quantity. Various modes of control may be utilized. One mode of control is known as pulse width modulation (PWM) peak current mode control. In PWM peak current mode control, the switch remains on until the current in the switch reaches a current limit. Once the current limit is reached, the controller turns the switch off for the remainder of the switching period. In general, a higher current limit results in a longer on-time of the switch and a bigger duty ratio. However, for controllers operating in continuous conduction mode (CCM) with large duty ratios (typically for duty ratios greater than 50%), small error signal perturbations may cause sub-harmonic oscillation to occur.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive embodiments of the invention are described with reference to the following figures, wherein like reference numerals refer to like parts throughout the various views unless otherwise specified.

FIG. 1 is a diagram illustrating an example switched mode power supply utilizing a controller, in accordance with embodiments of the present invention.

FIG. 2A is a diagram illustrating an example switching current waveform of the switched mode power supply of FIG. 1.

FIG. 2B is a diagram illustrating a further example of a switching current waveform of the switched mode power supply of FIG. 1 utilizing current mode pulse width modulation (PWM) control.

FIG. 3 is a diagram illustrating an example switching current waveform of FIG. 1 utilizing a control scheme in accordance with embodiments of the present invention.

FIG. 4 is a block diagram illustrating a controller in accordance with embodiments of the present invention.

FIG. 5A is a diagram illustrating an example voltage waveform utilizing a linear slope of the timing circuit of FIG. 4.

FIG. 5B is a diagram illustrating another example voltage waveform utilizing a non-linear slope of the timing circuit of FIG. 4.

FIG. 6 is a diagram illustrating various embodiments of the voltage waveform of FIG. 5.

FIG. 7 is a timing circuit in accordance with embodiments of the present invention.

FIG. 8 is a timing diagram illustrating various waveforms of voltages and currents of the timing circuit of FIG. 7.

DETAILED DESCRIPTION

Embodiments of a controller with a variable switching frequency and period are described herein. In the following description numerous specific details are set forth to provide a thorough understanding of the embodiments. One skilled in the relevant art will recognize, however, that the techniques described herein can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring certain aspects.

Reference throughout this specification to “one embodiment”, “an embodiment”, “one example” or “an example” means that a particular feature, structure or characteristic described in connection with the embodiment or example is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment”, “in an embodiment”, “one example” or “an example” in various places throughout this specification are not necessarily all referring to the same embodiment or example. Furthermore, the particular features, structures or characteristics may be combined in any suitable combinations and/or subcombinations in one or more embodiments or examples. In addition, it is appreciated that the figures provided herewith are for explanation purposes to persons ordinarily skilled in the art and that the drawings are not necessarily drawn to scale.

Various modes of control may be utilized to regulate the output of a power supply. One mode of control is known as pulse width modulation (PWM) current mode control. In PWM current mode control, the switch remains on until the current in the switch reaches a current limit or the maximum duty ratio has been reached. In one embodiment, the current limit is the peak current of the switch. Once the current limit is reached, the controller turns the switch off for the remainder of the switching period. In general, a higher current limit results in a longer on-time of the switch and a larger duty ratio. However, for controllers operating in continuous conduction mode (CCM) with large duty ratios (typically for duty ratios greater than 50%), small error signal perturbations may cause sub-harmonic oscillation to occur. In particular, sub-harmonic oscillation may occur for conventional PWM current mode control where the switching frequency (and therefore the switching period T_(S)) does not vary.

Typical methods to prevent sub-harmonic oscillation include varying the current limit with the duty ratio. In such a case, the current limit is not fixed and the current limit is a linearly decreasing ramp as the duty ratio increases. This is typically known as slope compensation. However, there are disadvantages to utilizing slope compensation. For example, in continuous conduction mode (CCM), output power is proportional to the peak current of the switch and the peak current decreases as the current limit linearly decreases. As a result, the output power would decrease for high duty ratios. Slope compensation also erodes the loop bandwidth and phase margin benefits of PWM current mode control. To offset the decrease in output power, the current limit may be increased overall. However, power supply components, such as the switch, transformer, clamp circuit, and output rectifier, would need to be rated for higher current values. This approach has its drawbacks since the higher the current rating for a component typically means an increase in the size of the component. As a result, utilizing current limit slope compensation would result in tradeoffs between size and output power.

With conventional PWM peak current mode control, where the switching frequency is not varied by the controller, sub-harmonic oscillation may occur at high duty ratios when in continuous conduction mode. As a result of sub-harmonic oscillation, the off-times of the switch may vary dramatically from one switching period to the next creating large ripple at the output voltage. Sub-harmonic oscillation may also reduce the maximum output power capability of the power supply. Thus, embodiments of the present invention reduce the likelihood of sustained sub-harmonic oscillation and the resultant large variations in off-time by varying the switching frequency (and therefore the switching period T_(S)) after some critical time t_(C) when sub-harmonic oscillation may occur. In one embodiment, the total switching period T_(S) is varied by a multiple of the difference between the on-time t_(ON) and the critical time t_(S). In a further embodiment, the off-time t_(OFF) of the switch is varied by a multiple of the difference between the on-time t_(ON) and the critical time t_(C). In one embodiment, the multiple is a fractional amount. In one example, this results in a substantially fixed off-time t_(OFF) over consecutive switching cycles. By varying the switching frequency when the on-time is greater than the critical time t_(C), the likelihood of sustained sub-harmonic oscillation is reduced. As will be further discussed, embodiments of the present invention include altering a timing circuit voltage to vary the switching frequency.

Referring first to FIG. 1, a diagram of an example switched mode power supply 100 is illustrated including input V_(IN) 102, an energy transfer element T1 104, a primary winding 106 of the energy transfer element T1 104, a secondary winding 108 of the energy transfer element T1 104, a switch S1 110, a clamp circuit 112, a rectifier D1 114, an output capacitor C1 116, a load 118, an output quantity U_(O), an output voltage V_(O), an output current I_(O), a feedback circuit 120, a controller 122, a feedback signal U_(FB) 124, a current sense input 126, a drive signal 128, and switch current I_(D) 130. The topology of the example switched mode power supply 100 illustrated in FIG. 1 is of the flyback regulator type, which is just one example of a switched mode power supply topology which may benefit from the teachings of the present invention. It is appreciated that other known topologies and configurations of switched mode power supply regulators may also benefit from the teachings of the present invention.

The switched mode power supply 100 provides output power to the load 118 from an unregulated input V_(IN) 102. In one embodiment the input V_(IN) 102 is a rectified and filtered ac line voltage. In another embodiment, the input voltage V_(IN) 102 is a dc input voltage. The input V_(IN) 102 is coupled to the energy transfer element T1 104. In some embodiments of the present invention the energy transfer element T1 104 may be a coupled inductor. In some other embodiments of the present invention the energy transfer element T1 104 may be transformer. In the example of FIG. 1, the energy transfer element T1 104 includes two windings, a primary winding 106 and secondary winding 108. N_(P) and N_(S) are the number of turns for the primary winding 106 and secondary winding 108, respectively. The primary winding 106 is further coupled to the active switch S1 110, which is then further coupled to the input return 111. In addition, the clamp circuit 112 is coupled across the primary winding 106 of the energy transfer element T1 104. The secondary winding 108 of the energy transfer element T1 104 is coupled to the rectifier D1 114. In the example illustrated in FIG. 1, the rectifier D1 114 is exemplified as a diode and the secondary winding 108 is coupled to the anode end of the diode. However, in some embodiments the rectifier D1 114 may be a transistor used as a synchronous rectifier. Both the output capacitor C1 116 and the load 118 are coupled to the rectifier D1 114. In the example of FIG. 1, the rectifier D1 114 is exemplified as a diode and both the output capacitor C1 116 and the load 118 are coupled to the cathode end of the diode. An output is provided to the load 118 and may be provided as either an output voltage V_(O), output current I_(O), or a combination of the two.

In addition, the switched mode power supply 100 further comprises circuitry to regulate the output which is exemplified as output quantity U_(O). In general, the output quantity U_(O) is either an output voltage V_(O), output current I_(O), or a combination of the two. A feedback circuit 120 is coupled to sense the output quantity U_(O). In one embodiment, the feedback circuit 120 may sense the output quantity U_(O) from the output of the power supply 100. In another embodiment, the feedback circuit 120 may sense the output quantity from an additional winding of the energy transfer element T1 104. Controller 122 is further coupled to the feedback circuit 120 and comprises several terminals. At one terminal, the controller 122 receives a feedback signal U_(FB) 124 from the feedback circuit 120. The controller 122 further includes terminals for the current sense input 126 and the drive signal 128. The current sense input 126 senses the switch current I_(D) 130 in switch S1 110. In addition, the controller 122 provides a drive signal 128 to the switch S1 110 to control various switching parameters. Examples of such parameters may include switching frequency, switching period, duty cycle, or respective on and off times of the switch S1 110.

In operation, the switched mode power supply 100 of FIG. 1 provides output power to the load 118 from an unregulated input V_(IN) 102, such as an unregulated input voltage. The switched mode power supply 100 utilizes the energy transfer element T1 104 to transform the voltage from the input V_(IN) 102 between the primary 106 and secondary 108 windings. The clamp circuit 112 is coupled to the primary winding 106 of the energy transfer element T1 104 to limit the maximum voltage on the switch S1 110. In one embodiment, the clamp circuit 112 limits the maximum voltage on the switch S1 110. Switch S1 110 is opened and closed in response to the drive signal 128 received from the controller 122. In some embodiments, the switch S1 110 may be a transistor and the controller 122 may include integrated circuits and/or discrete electrical components. In one embodiment controller 122 and switch S1 110 are included together into a single integrated circuit 132. In one example, integrated circuit 132 is a monolithic integrated circuit. In another example, integrated circuit 132 is a hybrid integrated circuit.

In operation, the switching of switch S1 110 produces a pulsating current at the rectifier D1 114. The current in rectifier D1 114 is filtered by output capacitor C1 116 to produce a substantially constant output voltage V_(O), output current I_(O), or a combination of the two at the load 118.

The feedback circuit 120 senses the output quantity U_(O) to provide the feedback signal U_(FB) 124 to the controller 122. In the example of FIG. 1, the controller 122 also receives the current sense input 126 which relays the sensed current I_(D) 130 in the switch S1 110. The switch current I_(D) 130 may be sensed in a variety of ways, such as for example the voltage across a discrete resistor or the voltage across the transistor when the transistor is conducting.

The controller 122 outputs a drive signal 128 to operate the switch S1 110 in response to various system inputs to substantially regulate the output quantity U_(O) to the desired value. With the use of the feedback circuit 120 and the controller 122, the output of the switched mode power supply 100 is regulated in a closed loop. In addition, the controller 122 includes a timing circuit (discussed in more detail below) which defines the switching cycle of the switch S1 110 with a switching period of T_(S) and a switching frequency of f_(S), where T_(S)=1/f_(S).

In one embodiment of the present invention, the controller 122 may utilize a control scheme which varies the switching frequency f_(S) when the on-time t_(ON) of the switch S1 110 is greater than a critical time t_(C). In a further embodiment, the controller 122 decreases the switching frequency f_(S) (or in other words, extends the switching period T_(S)) when the on-time t_(ON) of the switch S1 110 is greater than a critical time t_(C). For a fixed load, the switching period (and switching frequency) of the controller 122 may be a fixed period when the on-time is less than the critical time and the controller 122 varies the switching period (or switching frequency) when the on-time t_(ON) of the switch S1 110 is greater than a critical time t_(C).

As mentioned above, the switching period T_(S) may vary by some multiple of the difference between the on-time t_(ON) and the critical time t_(C). In some embodiments, the multiple is a value less than 1. By varying the switching frequency f_(S) when the switch has an on-time t_(ON) longer than a critical time t_(C), which corresponds to a large duty ratio, sub-harmonic oscillation may be prevented. In accordance with one embodiment of the present invention, sub-harmonic oscillation may be prevented by modulating the switching frequency f_(S) (and the switching period T_(S)) of switch S1 110 in response to the on-time t_(ON) of the switch S1 110. As will be further discussed, when the on-time t_(ON) of the switch S1 110 is greater than the critical time t_(C), a timing circuit included in the controller changes to an alternative discharging mode. Once the switch S1 110 turns off, the timing circuit resumes normal discharging mode. By selecting the rate of discharge of the timing circuit capacitor during the alternative discharging mode, sub-harmonic oscillation may be prevented.

The switching current of various conduction modes is illustrated in FIG. 2A. A diagram of an example switching current waveform of the power supply 100 of FIG. 1 is illustrated including a switching period T_(S) 204, a switch on-time t_(ON) 206, a switch off-time t_(OFF) 208, trapezoidal shape 210, and triangular shape 212. FIG. 2A illustrates the general waveforms of the switch current I_(D) 202 over time in both continuous conduction mode (CCM) and discontinuous conduction mode (DCM).

During any switching period T_(S) 204, switch S1 110 may conduct in response to the drive signal 128 from the controller 122 to regulate the output U_(O). The switching period T_(S) 204 may be separated into two sections of time: switch on-time t_(ON) 206 and switch off-time t_(Op) 208. Switch on-time t_(ON) 206 denotes the portion of the switching period T_(S) 202 which the switch S1 110 is conducting. Switch off-time I_(OFF) 208 denotes the remaining portion of the switching period T_(S) 202 when the switch S1 110 is not conducting. The current waveform of FIG. 2 shows two fundamental modes of operation. The trapezoidal shape 210 is characteristic of continuous conduction mode (CCM) whereas the triangular shape 212 is characteristic of discontinuous conduction mode (DCM). During CCM, the switch current I_(D) 202 is substantially non-zero immediately after the start of the switch on-time t_(ON) 206 and steadily increases throughout the switch on-time t_(ON) 208. At DCM, the switch current I_(D) 202 is substantially zero immediately after the beginning of the switch on-time t_(ON) 206 and steadily increases throughout the switch on-time t_(ON) 206. During the switch off-time I_(OFF) 204, the switch current I_(D) 202 is substantially zero for both CCM and DCM.

Sub-harmonic oscillation generally occurs when conventional controllers utilize PWM current mode control and operate in CCM at duty ratios greater than or equal to 50%. FIG. 2B demonstrates the general waveform of switch current I_(D) of FIG. 1 with respect to time when PWM current mode control of the switch S1 110 is used to regulate the output quantity U_(O). FIG. 2B illustrates switch current I_(D) 214, switching period T_(S) 216, switch on-time t_(ON) 218, switch off-time t_(OFF) 220, and a current limit I_(LIM) 222. In the example of FIG. 2B, the controller 122 is operating in CCM.

The switch S1 110 conducts at the beginning of each switching period T_(S) 216. Switch S1 110 conducts until the switch current I_(D) 214 reaches the current limit I_(LIM) 222. In one example, control of the current limit I_(LIM) 222 at a constant switching period T_(S) 216 (otherwise known as fixed switching frequency f_(S)) maintains the peak of the switch current I_(D) 214 at a value required to regulate the output quantity U_(O). In general, a higher current limit I_(LIM) 222 results in a longer switch on-time t_(ON) 218. In some embodiments of the present invention, the current limit I_(LIM) 222 is also the peak primary current I_(PEAK). The regulation is accomplished by a PWM technique known as fixed frequency PWM current mode control, fixed frequency PWM current programmed control, and/or peak current mode control.

It should be appreciated however, that the term “fixed frequency control” does not necessarily entail that the switching frequency f_(S) of the switch S1 110 remains unchanged. Instead, the use of the term “fixed frequency control” may merely indicate that the switching frequency f_(S) of the switch is not used as a control variable to regulate the output quantity U_(O). For the example of fixed frequency PWM current mode control, the value of the current limit I_(LIM) 222 is utilized as the control variable to regulate the output quantity U_(O). For various embodiments, varying the switching frequency f_(S) to prevent sub-harmonic oscillation may still be utilized along fixed frequency control modes since the switching frequency f_(S) is not utilized as a control variable to regulate the output quantity U_(O). In addition, frequency modulation for EMI emissions (also referred to as frequency jitter) may also be utilized with varying of the switching frequency to prevent sub-harmonic oscillation.

Referring next to FIG. 3, a diagram of example switching current waveform I_(D) of the power supply 100 utilizing a control scheme which varies the switching frequency f_(S) and the off-time t_(OFF) of the switch is illustrated including switch current I_(D) 302, switching periods T_(S) 304, 306, 308, and 310, base period T0 312, critical time t_(C) 314, a first current limit ILIM1 316, a second current limit ILIM2 318, a third current limit ILIM3 320, and a fourth current limit ILIM4 322. In addition, each switching period T_(S) 304, 306, 308, and 310 has a respective on-time t_(ON) and off-time t_(OFF). As shown in FIG. 3, switching periods T_(S) 308 and 310 also include a respective extended on-time t_(ONX), a base off-time I_(OFFB), and extension periods TX 324 and 326.

As mentioned above with respect to FIG. 2B, switch S1 110 conducts at the beginning of every switching period T_(S) until the current limit for the respective switching period T_(S) is reached. FIG. 3 illustrates the controller 122 operating in CCM and utilizing current mode control. As mentioned above, a larger current limit typically results in a longer on-time t_(ON). In general, how quickly the switch current I_(D) 302 increases to the current limit is partially dependant on the input voltage V_(IN) 102 and the inductance L_(p) of the primary winding 106.

During switching period T_(S) 304, the switch current I_(D) 302 increases until it reaches the first current limit I_(LIM1) 316. As illustrated, the on-time t_(ON) during the switching period T_(S) 304 is less than the critical time t_(C) 314. As a result, the switching period T_(S) 304 and the off-time t_(O) are not altered and as a result the switching period T_(S) 304 is fixed to be substantially equal to the base period T0 312. In one embodiment, critical time t_(C) 314 is substantially equal to one half the base period T0 312, or mathematically: t_(C)=½T0. The value of the critical time t_(C) is the point in time which sub-harmonic oscillation may occur if the switching frequency does not change.

During switching period T_(S) 306, the switch current I_(D) 302 increases until it reaches the second current limit I_(LIM2) 318. As illustrated, the on-time t_(ON) during switching period T_(S) 306 is substantially equal to critical time t_(C) 314 and the switching period T_(S) 306 and the off-time t_(O) are not altered. Thus, the switching period T_(S) 306 is also fixed to be substantially equal to the base period T0 312. When the on-time t_(ON) is substantially less than or equal to critical time t_(C) 314, as shown in switching periods T_(S) 304 and 306, the controller 122 is in a normal operating mode where switching periods T_(S) 304 and 306 are a fixed switching period. In one embodiment, a fixed switching period is a switching period that does not vary and is predetermined according to a set frequency of a timing circuit (e.g., an oscillator) included in controller 122.

However, during switching period T_(S) 308, the switch current I_(D) 302 increases until it reaches the third current limit I_(LIM3) 320. As illustrated, the on-time t_(ON) during switching period T_(S) 308 is greater than critical time t_(C) 314. In accordance with the embodiment of FIG. 3, the switching period T_(S) 308 is extended beyond the base period T0 312 by extension period TX 324. As will be further discussed, the length of extension period TX 324 is dependent on how long the on time t_(ON) extends past the critical time t_(C) 314. In other words, how much the switching period T_(S) 308 is extended depends on the difference between the on-time t_(ON) and the critical time t_(C) 314, also shown as extended on-time t_(ONX) in switching period T_(S) 308 (t_(ONX)=t_(ON)−t_(C)).

During switching period T_(S) 310, the switch current I_(D) 302 increases until it reaches the fourth current limit I_(LIM4) 322. As illustrated, the on-time t_(ON) during switching period T_(S) 310 is greater than critical time t_(C) 314 and the switching period T_(S) 310 and as a result is extended beyond the base period T0 312 by extension period TX 326. The length of the extension period TX 326 is dependent on the extended on-time t_(ONX) during switching period T_(S) 310.

As illustrated by switching periods 308 and 310, the switching period T_(S) may be expressed as a function of the base period T0 312 and the extension period TX: T _(S) =T0+TX  (1) The length of the extension period TX is dependent on the extended on-time t_(ONX) during the respective switching period T_(S). As described above with reference to switching periods 304 and 306, the switching period is fixed and substantially equal to the base period T0 312 when the on-time t_(ON) is less than or equal to the critical time t_(C) 314. Also, the extension periods TX 324 and 326 may be written as: TX=k(t _(ON) −t _(C))=kt _(ONX)  (2) Where k is an extension coefficient and 0≦k. In one embodiment, the extension coefficient k is a constant. In another embodiment, the extension coefficient k is not constant (as will be further discussed). It should be appreciated, that an extension in the switching period T_(S) results in a decrease in the switching frequency f_(S). In one embodiment, the value of the extension coefficient k may depend on various parameters of the controller 122 (such as parameters of a timing circuit of the controller 122, extended on-time t_(ONX), base period T0, on-time t_(ON), or the duty ratio D). By determining the value of the extension coefficient k, sub-harmonic oscillation may be prevented. In other words, by determining how much the extended on-time t_(ONX) affects the switching period T_(S), sub-harmonic oscillation may be prevented. As illustrated by equations 1 and 2, the switching period T_(S) may vary by a multiple of the difference between the on-time t_(ON) and the critical time t_(C) 314.

Further, the off-time t_(OFF) may vary from the base off-time t_(OFFB) and the amount of variation is also dependant on the difference between the on-time t_(ON) and the critical time t_(C) 314 (t_(ONX)). As illustrated, the off-time t_(OFF) may be expressed as a function of the base off-time t_(OFFB) and extension period TX when the on-time t_(ON) is greater than the critical time t_(C) 314: t _(OFF) =t _(OFFB) +TX  (3) where the base off-time t_(OFFB) is the difference between the base period T0 312 and the on-time t_(ON):t_(OFFB)=T0−t_(ON). In other words, the base off-time t_(OFFB) represents the value of the off-time t_(OFF) if the switching period T_(S) was not extended past the base period T0 312 in accordance with embodiments. It should be noted that the off-time t_(OFF) is substantially equal to the base off-time t_(OFFB) when the on-time t_(ON) is less than or equal to the critical time t_(C) 314. By substituting equation (2) into equation (3), the off-time may be expressed as: t _(OFF) =t _(OFFB) +kt _(ONX)  (4) As shown by equation 4, the off-time t_(OFF) may vary by a multiple of the difference between the on-time t_(ON) and the critical time t_(C) 314.

Further, sub-harmonic oscillation may also be prevented by ensuring the off-time t_(OFF) is greater than or equal to a limit determined by the base period T0 and the on-time t_(ON):

$\begin{matrix} {t_{OFF} \geq \frac{T\; 0^{2}}{4t_{ON}}} & (5) \end{matrix}$ Utilizing equation (5), the switching period T_(S) may be expressed as:

$\begin{matrix} {T_{S} \geq {t_{ON} + \frac{T\; 0^{2}}{4t_{ON}}}} & (6) \end{matrix}$

Utilizing equations (5) and (6), the switching period T_(S) may then be manipulated and expressed as a function of the base period T0, the extended on-time t_(ONX), and the extension coefficient k such as equations (1) and (2):

$\begin{matrix} {T_{S} \geq {{T\; 0} + {\left( \frac{t_{ONX}}{\frac{T\; 0}{2} + t_{ONX}} \right)t_{ONX}}}} & (7) \end{matrix}$ From equation (7), the extension coefficient, k is a function of the extended on-time t_(ONX) and the base period T0:

$k = {\frac{t_{ONX}}{\frac{T\; 0}{2} + t_{ONX}}.}$ As such, in one embodiment, sub-harmonic oscillation may be prevented when the switching period T_(S) is greater than or equal to the quantity illustrated in equation (7).

Referring next to FIG. 4, a block diagram of an example of controller 122 is illustrated including a pulse width modulation (PWM) block 402 and a timing circuit 404. The PWM block 402 includes a comparator 406, an OR gate 408, and a flip-flop 412. Further illustrated in FIG. 4 is the feedback circuit 120, feedback signal U_(FB) 124, current sense signal 126, drive signal 128, DCMAX signal 410, clock signal 416, and on-time signal U_(ON) 418.

The controller 122 includes PWM block 402 and timing circuit 404. PWM block 402 is coupled to receive the current sense signal 126 and feedback signal U_(FB) 124. The PWM block 402 is also coupled to the timing circuit 404 to receive the clock signal 416. Optionally, the PWM block 402 may also receive DCMAX signal 410 from the timing circuit 404. Utilizing the clock signal 416, DCMAX signal 410, current sense signal 126 and the feedback signal U_(FB) 124, the PWM block 402 outputs the drive signal 128.

PWM block 402 further includes comparator 406, OR gate 408, and flip-flop 412. The comparator 406 is coupled to receive current sense signal 126 and feedback signal U_(FB) 124. In the example shown, the current sense signal 126 is received at the non-inverting input of comparator 406 while the feedback signal U_(FB) 124 is received at the non-inverting input of comparator 406. In one embodiment, the feedback signal U_(FB) 124 is a voltage signal or a current signal and may be representative of the current limit of the switch S1 110. In another embodiment, comparator 406 receives a variable current limit that is determined responsive to a value of feedback signal U_(FB) 124. Further, the current sense signal 126 is a voltage signal or a current signal and is representative of the switch current I_(D) 130. When the value of the current sense signal 126 is greater than the value of the current limit provided by the feedback signal U_(FB) 124, the output of comparator 406 is logic high. Otherwise, the output of comparator 406 is logic low.

The output of comparator 406 couples to one input of OR gate 408. The other input of OR gate 408 is coupled to receive the DCMAX signal 410.

DCMAX signal 410 is a rectangular waveform with varying lengths of logic high and logic low sections. In one example, the falling edge of the logic high section corresponds to the maximum duty ratio D_(MAX). In another example, the length of the logic high section of DCMAX signal 410 corresponds to the maximum duty ratio. However, the small circle at the input of the OR gate 408 indicates that the OR gate 408 receives the inverted DCMAX signal 410.

The flip-flop 412 couples to OR gate 408 and to timing circuit 404. In the example shown, the flip-flop 412 is D flip-flop and the timing circuit 404 is coupled to provide the clock signal 416 to the clock-input of flip-flop 412. The clock signal 416 is a rectangular pulse waveform and the amount of time between consecutive rising edges is substantially equal to the switching period T_(S). Further, the output of OR gate 408 is coupled to the reset-input of flip-flop 412. As shown, the D-input of flip-flop 412 is coupled to receive a logic high value. The flip-flop 412 then outputs the drive signal 128 to the switch S1 110. Drive signal 128 is a rectangular waveform with varying lengths of logic high and logic low sections. In one embodiment, the logic high sections correspond to an on-time of switch S1 110 while the logic low sections correspond to an off-time of switch S1 110.

The timing circuit 404 receives the on-time signal U_(ON) 418 and outputs the clock signal 416 to the PWM block 402. Optionally, the timing circuit 404 may also provide the DCMAX signal 410 to the PWM block 402. On-time signal U_(ON) 418 provides information regarding the on-time of the switch S1 110 and may be a rectangular pulse waveform with varying lengths of logic high and logic low sections. In one example, the drive signal 128 may be utilized for the on-time signal, such that the timing circuit 404 is coupled to an output of flip-flop 412 to receive on-time signal U_(ON) 418A. Alternatively, the current sense signal 126 may be utilized for the on-time signal U_(ON) 418, such that the timing circuit 404 is coupled to receive on-time signal U_(ON) 418B. In yet another example, the output of comparator 406 may be utilized for the on-time signal U_(ON) 418, such that the timing circuit 404 is coupled to the output of comparator 406 to receive on-time signal U_(ON) 418C.

The timing circuit 404 provides the switching period T_(S) to the PWM block 402 via the clock signal 416. That is, in one example, the period of clock signal 416 is the switching period T_(S). In one example, an oscillator may be utilized for the timing circuit 404. Utilizing the on-time t_(ON) of the switch S1 110 provided by the on-time signal U_(ON) 418, the timing circuit 404 varies the switching period T_(S) by a multiple of the difference between the on-time t_(ON) and the critical time t_(C). In embodiments, the timing circuit 404 does not vary the switching period T_(S) unless the on-time t_(ON) is greater than the critical time t_(C). In one embodiment, the clock signal 416 is a rectangular pulse waveform. In one embodiment, the rising edge of the clock signal 416 indicates the beginning of switching period T_(S).

In operation, when the clock signal 416 pulses to a logic high value signaling the beginning of a switching period T_(S). The output of the flip-flop 412 transitions to a logic high value (due to the logic high at the D-input) and the drive signal 128 turns the switch S1 110 on. In one embodiment, clock signal 416 falls to a logic low value when the critical time t_(C) is reached and the output of the latch 412 remains at the logic high value until the flip-flop 412 is reset. If either the output of comparator 406 is logic high (corresponding to when the value of the current sense signal 126 is greater than the value of the current limit provided by the feedback signal U_(FB) 124) or the inverted DCMAX signal 410 is logic high (or both), the output of OR gate 408 is logic high. When the flip-flop 412 receives a logic high value at the reset-input, the drive signal 128 (i.e. output of the flip-flop 412) transitions to a logic low value and the switch S1 110 is turned off. Examples of timing waveforms for the clock signal 416 and DCMAX signal 410 will be discussed with respect to FIG. 8.

Referring next to FIG. 5A, a diagram illustrating voltage waveform VTIM 528 of the timing circuit 404 of FIG. 4 is shown including a lower reference voltage V_(L) 530, an upper reference voltage V_(H) 532, a max duty ratio voltage V_(DM) 533, and critical time t_(C) 514. Further illustrated in FIG. 5A is switch current I_(D) 502, switching periods T_(S) 504, 506, 508, and 510, base period T0 512, critical time t_(C) 514, a first current limit ILIM1 516, a second current limit ILIM2 518, a third current limit ILIM3 520, and a fourth current limit ILIM4 522. In addition, each switching period T_(S) 504, 506, 508, and 510 has a respective on-time t_(ON) and off-time I_(DFF). As shown in FIG. 5A, switching periods T_(S) 508 and 510 also include a respective extended on-time t_(ONX), a base off-time I_(DFFB), and extension periods TX 524 and 526.

The waveform representing the switch current I_(D) 502 is similar to the switch current I_(D) 302 illustrated in FIG. 3 along with corresponding reference numbers. FIG. 5A illustrates one example of a timing voltage VTIM 528 for the respective switch current I_(D) 502. In one embodiment, timing circuit 404 includes a timing capacitor (discussed below), where the timing voltage VTIM 528 is the voltage across the timing capacitor. In each switching period, the timing voltage VTIM 528 increases with slope m₁ until it reaches the upper reference voltage V_(H) 532 and then decreases until it reaches lower reference voltage V_(L) 530. However, timing circuit 404 may include two modes for discharging the capacitor to the lower reference voltage V_(L) 530. In a normal discharging mode the timing voltage VTIM 528 decreases with slope m₃ until the lower reference voltage V_(L) 530 is reached. When the on-time t_(ON) is greater than the critical time t_(C), the timing circuit 404 switches to an alternative discharging mode and the timing voltage VTIM 528 discharges with two or more slopes (e.g., m₂ and m₃) that are either negative or zero. In one, embodiment, at the end of on-time t_(ON), the timing circuit 404 changes from the alternative discharging mode to the normal discharging mode and the timing voltage VTIM 528 decreases with slope m₃. In addition, the upper reference voltage V_(H) 532 may correspond to the critical voltage V_(c). The critical voltage V_(c) indicates when the timing circuit is at 50% of the base period T0 512. In other words, at critical time t_(C) 514, the value of timing voltage VTIM 528 is substantially equal to the upper reference voltage V_(H) 532. As shown, the timing voltage VTIM 528 utilizes three voltage thresholds.

At the beginning of each switching period, the timing voltage VTIM 528 begins at the lower reference voltage V_(L) 530 and increases to the upper reference voltage V_(H) 532. Once at the upper reference voltage V_(H) 532, the timing voltage VTIM 528 decreases until it reaches the lower reference voltage V_(L) 530. When the timing voltage VTIM 528 reaches the lower reference voltage V_(L) 530, the current switching period has ended and a new switching period has begun. As such, the time for the timing voltage VTIM 528 to rise to the lower reference voltage V_(L) 530 and subsequently decrease to the upper reference voltage V_(H) 532 determines the length of the switching period T_(S). In other words, the switching period T_(S) of clock signal 416 is equal to a charging time that the timing capacitor charges to an upper reference voltage V_(H) 532 plus a discharging time that the timing capacitor discharges to a lower reference voltage V_(L) 530.

During switching period T_(S) 504, the on-time t_(ON) is less than the critical time t_(C) 514 and the switching period T_(S) 504 is not extended beyond the base period T0 512. As a result, the timing circuit 404 operates in a normal discharging mode. As shown by FIG. 5A, the timing voltage VTIM 528 rises to the upper reference voltage V_(H) 532 with slope m₁ and falls to the lower reference voltage V_(L) 530 with slope m₃ without interruption. In one embodiment, the magnitude of slope m₃ is substantially equal to the magnitude of slope m₁, or mathematically: |m₃|=|m₁|.

During switching period T_(S) 506, the on-time t_(ON) is substantially equal to the critical time t_(C) 514 and the switching period T_(S) 506 is not extended beyond the base period T0 512. Similar to switching period T_(S) 504, the timing circuit 404 operates in a normal discharging mode and the timing voltage VTIM 528 rises to the upper reference voltage V_(H) 532 with slope m₁ and falls to the lower reference voltage V_(L) 530 with slope m₃ without interruption.

During switching period T_(S) 508, the on-time t_(ON) is greater than the critical time t_(C) 514. As a result, the timing circuit 404 operates in an alternative discharging mode and the switching period T_(S) 504 is extended beyond the base period T0 512. In the example of FIG. 5A, the timing voltage waveform VTIM 528 rises with slope m₁ until the on-time t_(ON) is substantially equal to the critical time t_(C) 514. In other words, the timing voltage waveform VTIM 528 increases with slope m₁ until the upper reference voltage V_(H) 532 is reached. When the upper reference voltage V_(H) 532 is reached, the timing circuit 404 switches to an alternative discharging mode and the timing voltage VTIM 528 will decrease with slope m₂. As further shown in FIG. 5A, the upper reference voltage V_(H) 532 corresponds to the value of the timing voltage VTIM 528 when the on-time t_(ON) is substantially equal to the critical time t_(C) 514. In other words, the time at which the timing voltage VTIM 528 reaches the upper reference voltage V_(H) 532 is the time at which the on time t_(ON) reaches the critical time t_(C) 514, and the timing circuit 404 then switches to an alternative discharging mode where the timing voltage VTIM 528 decreases with slope m₂.

In the example shown in FIG. 5A, the slope m₂ is less than slope m₃. In one embodiment, slope m₂ is equal to one half of slope m₃ or mathematically: m₂=½m₃. As will be further discussed, the ratio between slope m₂ and slope m₃ (and subsequently slope m₁) may alternatively be expressed in terms of the duty ratio. The timing voltage VTIM 528 will decrease with slope m₂ until the switch S1 110 is turned off. In other words, during the extended on-time t_(ONX), the rate of decrease of the timing voltage waveform VTIM 528 is determined by slope m₂. Once the switch S1 110 is turned off, the timing voltage waveform VTIM 528 decreases with slope m₃ until the lower reference voltage V_(L) 530 is reached. FIG. 5A illustrates that slope m₂ is constant, resulting in a linear decrease of the timing voltage VTIM. In another embodiment, slope m₂ is not constant, resulting in a non-linear decrease of the timing voltage VTIM.

During switching period T_(S) 510, the on-time t_(ON) is greater than the critical time t_(C) 514. However, the on-time t_(ON) during switching period T_(S) 510 is longer than the on-time t_(ON) during switching period T_(S) 508. In other words, the extended on-time t_(ONX) during switching period T_(S) 510 is longer than the extended on-time t_(ONX) during switching period T_(S) 508. Similar to the previous switching period, the timing voltage VTIM 528 rises with slope m₁ to the upper reference voltage V_(H) 532. The timing voltage VTIM 528 then decreases with slope m₂ for the remainder of the on-time t_(ON). Once the switch S1 110 is turned off, the timing voltage VTIM 528 decreases further with slope m₃ to the lower reference voltage V_(L) 530.

Further illustrated in FIG. 5A is max duty ratio voltage V_(DM) 533. As will be further discussed if the timing voltage waveform VTIM 528 falls below the max duty ratio voltage V_(DM) 533 after the timing voltage waveform VTIM 528 has reached the upper reference voltage V_(H) 530 and before the switch S1 110 is turned off, the DCMAX signal 410 is enabled and the switch S1 110 is turned off. Fixing the max duty ratio voltage V_(DM) 533 ensures a minimum off-time t_(OFF) of the switch S1 110 and further prevents sub-harmonic oscillation.

As mentioned above with respect to FIG. 3, the switching period T_(S) may be expressed in terms of the base period T0 512 and the extension period TX. Further, the extension period TX may be expressed in terms of the extended on-time t_(ONX). By combining equations (1) and (2), the switching period T_(S) may be expressed as: T _(S) =T0+kt _(ONX)  (8) where t_(ONX)=t_(ON)−t_(C). As mentioned above, the value of the extension coefficient k may be partially determined by the properties of the controller 122 and the timing circuit 404. For a timing circuit 404 with an alternative discharging mode, once the on-time t_(ON) is greater than the critical time t_(C), the extension coefficient k is proportional to the ratio of slope m₂ to slope m₃. For example, the extension coefficient k may be the difference between the value of one and the ratio of slope m₂ to slope m₃, expressed as:

$\begin{matrix} {k = {1 - {\frac{m_{2}}{m_{3}}}}} & (9) \end{matrix}$ However, since |m₃|=|m₁|, the extension coefficient may also be expressed in terms of slope m₂ and slope m₁. By combining equation (8) and equation (9) and expressing equation (9) in terms of slope m₁, the switching period T_(S) may be expressed as:

$\begin{matrix} {T_{S} = {{T\; 0} + {\left( {1 - {\frac{m_{2}}{m_{1}}}} \right)t_{ONX}}}} & (10) \end{matrix}$ As illustrated in equation (10), the switching period T_(S) may vary by some multiple of the difference between the on-time t_(ON) and the critical time t_(C). Further, the switching period T_(S) can also be expressed in terms of the duty ratio D, slope m₁, slope m₂, and the base period T0:

$\begin{matrix} {T_{S} = \frac{T\; 0\left( {1 + {\frac{m_{2}}{m_{1}}}} \right)}{2\left( {1 - D + {D{\frac{m_{2}}{m_{1}}}}} \right)}} & (11) \end{matrix}$ As mentioned above, the compensation coefficient k may equal:

$k = {\frac{t_{ONX}}{\frac{T\; 0}{2} + t_{ONX}}.}$ By comparing equation (10) with equation (7), we can determine a boundary relationship for slope m₂ and m₁. For stability:

$\begin{matrix} {{1 - {\frac{m_{2}}{m_{1}}}} \geq \frac{t_{ONX}}{\frac{T\; 0}{2} + t_{ONX}}} & (12) \end{matrix}$ Which can be simplified to:

$\begin{matrix} {{\frac{m_{2}}{m_{1}}} \leq \frac{1}{1 + \frac{2\; t_{ONX}}{T\; 0}}} & (13) \end{matrix}$ In one embodiment, sub-harmonic oscillation may be prevented by utilizing equation (13) to determine the ratio between slope m₂ and slope m₁.

In another embodiment of the present invention, for a constant slope m₂, the ratio of slope m₂ and m₁ is partially determined by the maximum duty ratio:

$\begin{matrix} {{\frac{m_{2}}{m_{1}}} \leq \frac{1 - D_{MAX}}{D_{MAX}}} & (14) \end{matrix}$ As such, the boundary equation for the compensation coefficient k of equation (9) may be expressed as:

$\begin{matrix} {k \geq \frac{{2\; D_{MAX}} - 1}{D_{MAX}}} & (15) \end{matrix}$ For the example shown in of FIG. 5A, the maximum duty ratio is substantially 66%. By utilizing equations (14) and (15) the slope m₂ is substantially one-half the magnitude of slope m₁ and the extension coefficient k is substantially one-half. As such, the extension period TX (the amount of time which the switching period T_(S) is extended beyond the base period T0 512) is substantially one half of to the extended on-time t_(ONX).

In another example, the slope m₂ is substantially equal to zero. When slope m₂ is zero, the extension coefficient k is substantially one. As such, in this example, the extension period TX is substantially equal to the extended on-time t_(ONX). As will be further discussed with respect to FIG. 5B, the slope m₂ may vary as a function of the duty ratio and/or on-time t_(ON).

FIG. 5B illustrates a similar voltage waveform VTIM 528 as FIG. 5A, however, the slope m₂ is a variable slope. As illustrated in switching periods T_(S) 509 and 511, the voltage waveform VTIM 528 rises with slope m₁ until the on-time t_(ON) is substantially equal to the critical time t_(C) 514. Once the critical time t_(C) 514 is reached, the timing voltage VTIM 528 decreases with an adaptable slope m₂ for the remainder of the on-time t_(ON). Once the switch S1 110 is turned off, the timing voltage VTIM 528 decreases with slope m₃ until the lower reference voltage V_(L) 530 is reached. However, the slope m₂ is variable. In one embodiment, the slope m₂ may vary as a function of the duty ratio D.

As mentioned above with regards to equation (14), when slope m₂ is constant, the ratio between slope m₂ and slope m₁ may be expressed in terms of the maximum duty ratio

${D_{MAX}\text{:}\mspace{14mu}{\frac{m_{2}}{m_{1}}}} \leq {\frac{1 - D_{MAX}}{D_{MAX}}.}$ However, by varying the slope m₂ within each switching period, the switching frequency f_(S) may vary less from the base frequency than if the slope m₂ was kept constant. In other words, by varying the slope m₂, there is less of a reduction in the switching frequency than if a constant slope m₂ was utilized. Thus, in one embodiment, the ratio between the slope m₂ and slope m₁ may be expressed in terms of the duty ratio:

$\begin{matrix} {{\frac{m_{2}}{m_{1}}} = \frac{1 - D}{D}} & (16) \end{matrix}$ Equation (9) illustrates that the extension coefficient k may be expressed as a function of slope m₂ and slope m₁. As such, the extension coefficient k may also be variable as a function of the duty ratio D. Further, by substituting equation (16) into equation (10) above, the switching period T_(S) may be expressed in terms of the duty ratio D and the base period T0 when slope m₂ is non-zero:

$\begin{matrix} {T_{S} = \frac{T\; 0}{4\left( {1 - D} \right)D}} & (17) \end{matrix}$

In yet another embodiment, slope m₂ is equal to zero, where equation (17) can be further simplified to:

$\begin{matrix} {T_{S} = \frac{T\; 0}{2\left( {1 - D} \right)}} & (18) \end{matrix}$

Referring next to FIG. 6, a diagram illustrating various embodiments of the timing voltage waveform VTIM 602 with slope m₂ of timing circuit 404 is shown including a lower reference voltage V_(L) 604, an upper reference voltage V_(H) 606, a max duty ratio voltage V_(DM) 607, a critical time t_(C) 608, an on-time t_(ON) 610, an extended on-time t_(ONX) 612, a base off-time t_(OFFB 6) 14, a slope m₁ 616, a slope m₂ 618 and a slope m₃ 619. Illustrated is timing waveforms 622, 624, and 626. FIG. 6 illustrates the changes to the respective switching period T_(S) and off-time t_(OFF) of each timing waveform 622, 624, and 626 with varying values of slope m₂ 618. Further included in FIG. 6 is waveform 620 (in dashed lines) which illustrates the properties of the timing circuit 404 if the switching period T_(S) and frequency f_(S) were not varied when the on-time t_(ON) 610 is greater than the critical time t_(C) 608. The various embodiments of the timing voltage waveform VTIM 602 illustrated in FIG. 6 follow similar properties as the voltage waveform VTIM 528 illustrated in FIG. 5.

As shown in FIG. 6, the timing voltage VTIM 602 increases until the upper reference voltage V_(H) 606. Once the upper reference voltage V_(H) 606 is reached the timing voltage VTIM 602 decreases until the lower reference voltage V_(L) 604 is reached. The time it takes for the timing voltage VTIM 602 to reach the upper reference voltage V_(H) 606 and then fall to the lower reference voltage V_(L) 604 is substantially the switching period T_(S) for the particular switching cycle. In the example shown in FIG. 6, the timing circuit 404 alters the switching period or switching frequency by altering the rate of decrease to the lower reference voltage V_(L) 604.

Further, the controller 122, and subsequently the timing circuit 404, switches to an alternative discharging mode when the on-time t_(ON) 610 is greater than the critical time t_(C) 608. In other words, the switching period and switching frequency are varied once the on-time t_(ON) 610 is greater than the critical time t_(C) 608. In addition, the upper reference voltage V_(H) 606 is chosen such that, when the timing voltage VTIM 602 reaches the upper reference voltage V_(H) 606, the on-time t_(ON) 610 substantially equal to the critical time t_(C) 608. In one embodiment, the controller 122, and subsequently the timing circuit 404, may switch to an alternative discharging mode when the timing voltage VTIM 602 reaches the upper reference voltage V_(H) 606. As will be illustrated, how much the switching period and switching frequency are varied depends partially on the properties of the alternative discharging mode of timing circuit 404.

Waveforms 622, 624, and 626 each illustrate the timing voltage VTIM 602 increasing at the beginning of on-time t_(ON) 610 with slope m₁ 616 until the upper reference voltage V_(H) 606 (corresponding to the on-time t_(ON) 610 is substantially equal to the critical time t_(C) 608). Once the upper reference voltage V_(H) 606 is reached, the timing voltage VTIM 602 switches to an alternative discharging mode if the switch S1 110 is still on. In other words, timing voltage VTIM 602 decreases with slope m₂ 618 for the remainder of the on-time t_(ON) 610 that extends past the critical time t_(C) 608, otherwise referred to as extended on-time t_(ONX) 612. As mentioned above, in one embodiment the magnitude of slope m₃ 619 is substantially equal to the magnitude of slope m₁ 616.

However, waveform 620 illustrates a timing voltage of a conventional controller that does not include an alternative discharging mode as disclosed herein. As a result, the switching period T_(S) for waveform 620 is substantially equal to the base period T0 628. In addition, the off-time for waveform 620 is substantially equal to the base off-time t_(OFFB) 614. As shown in FIG. 6, the resultant waveform 620 is the same as the timing waveform when the on-time t_(ON) is less than or equal to the critical time t_(C). Thus, sub-harmonic oscillation may still occur when duty ratios are greater than 50%.

For waveform 622, slope m₂ 618 is substantially equal one-half slope m₃ 619 and one-half slope m₁ 616, or mathematically: |m₂|=½|m₃|; |m₂|=½|m₁|. As a result (and further shown by equation (10)), the switching period T1 630 of waveform 622 is extended past the base period T0 628 by one-half the extended on-time t_(ONX) 612, or mathematically: T1=T0+½t_(ONX). In addition, the off-time t_(OFFB) of waveform 622 is also extended past the base off-time t_(OFFB) 614 by one-half the extended on-time t_(ONX) 612.

For waveform 624, slope m₂ 618 is substantially equal one-quarter slope m₃ 619 and one-quarter slope m₁ 616, or mathematically: |m₂|=¼|m₃|; |m₂|=¼|m₁|. As a result (and further shown by equation (10)), the switching period T2 632 of waveform 624 is extended past the base period T0 628 by three-quarters of the extended on-time t_(ONX) 612, or mathematically: T1=T0+¾t_(ONX). In addition, the off-time t_(OFF2) of waveform 624 is also extended past the base off-time t_(OFFB) 614 by three-quarters the extended on-time t_(ONX) 612.

For waveform 626, slope m₂ 618 is substantially equal to zero, or mathematically: m₂=0. As a result (and further shown by equation (10)), the switching period T3 634 of waveform 626 is extended past the base period T0 628 by the extended on-time t_(ONX) 612, or mathematically: T1=T0+t_(ONX). The off-time t_(OFF3) of waveform 626 is also extended past the base off-time t_(OFFB) 614 by the extended on-time t_(ONX) 612. In particular, when slope m₂ 618 is substantially equal to zero, the off-time t_(OFF3) is substantially fixed for each switching cycle.

For waveforms 622, 624, and 626, once the switch S1 110 turns off and the off-time begins, the timing circuit 404 (and as a result the timing voltage VTIM 602) returns to a normal discharging mode. As discussed above, the timing voltage VTIM 602 decreases with slope m₃ 619 until the lower reference voltage V_(L) 604 is reached.

By selecting the value of slope m₂ 618, sub-harmonic oscillation may be prevented. As mentioned above, when the ratio between the magnitude of slope m₂ 618 and the magnitude of slope m₁,

${{\frac{m_{2}}{m_{1}}} \leq \frac{1 - D}{D}},$ sub-harmonic oscillation may be prevented. In one embodiment, slope m₂ 618 is one-half of slope m₁ 616. As shown in FIG. 6, the smaller the value of slope m₂ 618 with respect to slope m₁ 616, the longer the switching period T_(S) is extended beyond the base period T0 628. However, output power of the power supply 100 is proportional to the switching frequency f_(S). The longer the switching period T_(S), the smaller the switching frequency f_(S), which may result in a drop in the output power.

As mentioned above, frequency modulation for EMI emissions (also referred to as frequency jitter) may also be utilized with varying of the switching frequency. In one embodiment, frequency jitter may be accomplished by varying the slope m₁ 616.

Referring next to FIG. 7, an example timing circuit 704 which may be utilized as timing circuit 404 of FIG. 4 is illustrated including an upper reference voltage V_(H) 701, a lower reference voltage V_(L) 702, a max duty ratio voltage V_(DM) 705, a comparator 706, a latch 710, a current source 712 with charge current I_(C), a current sink 714 with discharge current I_(DIS) a current sink 716 with extension current I_(EXT), and capacitor 718 (i.e., a timing capacitor) with timing voltage V_(TIM). The timing circuit 704 further includes AND gates 720 and 722, a comparator 726, inverters 728 and 730, and a latch 732. Further illustrated in FIG. 7 are on-time signal U_(ON) 418, DCMAX signal 410, clock signal 416, critical signal (CRT) 734, and extension signal (EXT) 736. Further shown are switches S5, S6, S7, S8, S9 and S10.

The current source 712 charges capacitor 718 (i.e., a timing capacitor) with charge current I_(C) to an upper reference voltage V_(H) 701. The magnitude of the charge current I_(C) partially determines the value of slope m₁, discussed with respect to FIGS. 5A, 5B and 6. Once the timing voltage V_(TIM) of the capacitor 718 reaches the upper reference voltage V_(H) 701, the capacitor 718 is discharged through current sinks 714 and 716 with discharge current I_(DIS) and extension current I_(EXT), respectively, until the timing voltage V_(TIM) across the capacitor 718 reaches the lower reference voltage V_(L) 702. The magnitudes of the discharge current I_(DIS) and the extension current I_(EXT) partially determine the value of slopes m₂ and m₃. In one embodiment, an additional current source (not shown) coupled to charge capacitor 718, in addition to current source 712, may be included to implement frequency jitter. In this example, the additional current source would provide a triangular current waveform (jitter current) to vary the value of slope m₁.

The difference between the upper reference voltage V_(H) 701 and the lower reference voltage V_(L) 702 is referred herein as the amplitude swing of the timing circuit 404. In one embodiment, the amplitude swing of the timing circuit 404 is fixed. When the amplitude swing is fixed, the time it takes for the voltage V_(TIM) of capacitor 718 to charge to the upper reference voltage V_(H) 701 and discharge to the lower reference voltage V_(L) 702 determines the frequency and period of timing circuit 404. The timing voltage V_(TIM) of capacitor 718 linearly increases and decreases depending on the value of the charge current I_(C), extension current I_(EXT), and discharge current I_(DIS). In other words, the magnitudes of the charge current I_(C), extension current I_(EXT), and discharge current I_(DIS) may determine the frequency of timing circuit 404 and therefore vary the switching frequency f_(S) and switching period T_(S) of switch S1 110.

As mentioned above, in embodiments the timing circuit 404 varies the switching frequency f_(S) and switching period T_(S) when the on-time t_(ON) of the switch S1 110 is greater than a critical time t_(C). As will be further discussed, in one example timing circuit 404 utilizes the extension current I_(EXT) to vary the frequency of timing circuit 404 and therefore vary the switching frequency f_(S) and switching period T_(S) of switch S1 110 when the on-time t_(ON) of the switch S1 110 is greater than a critical time t_(C).

Capacitor 718 is coupled to comparator 706 such that the timing voltage V_(TIM) is received at the non-inverting terminal of comparator 706. Further, the inverting terminal of comparator 706 may received either the lower reference voltage V_(L) 702 or the upper reference voltage V_(H) 701, depending on whether switch S9 or switch S10 is closed. The timing voltage V_(TIM) waveform is illustrated in FIG. 8 as waveform VTIM. Further examples of the timing voltage waveform V_(TIM) may be found with respect to FIGS. 5A, 5B and 6. The output of comparator 706 is also coupled to control switching of the switches S5, S6 and S7. As shown in FIG. 7, the output of comparator 706 may also be referred to as critical signal CRT 734. Since the upper reference voltage V_(H) 701 corresponds to the value of the timing voltage V_(TIM) when a time period substantially equal to the critical time t_(C) has passed, the output of comparator 706 provides information regarding the critical time t_(C). The critical signal CRT 734 (output of comparator 706) is coupled to control switching of switch S7 and S6. However, inverter 730 is coupled between the comparator 706 and switch S5. As such, the inverted critical signal CRT is coupled to control switching of switch S5. In other words, when the critical signal CRT 734 is logic low switch S5 is closed (i.e. ON) and switches S6 and S7 are open (i.e. OFF). When the critical signal CRT 734 is logic high, switch S5 is open (i.e. OFF) while switches S6 and S7 are closed (i.e. ON).

Current source 712 charges capacitor 718 with charge current I_(C) when switch S5 is on and switches S6, S7, and S8 are off. Capacitor 718 is discharged through current sink 716 with extension current I_(EXT) when switch S7 is on. The capacitor 718 is further discharged through current sink 714 with discharge current I_(DIS) when both switches S6 and S8 are on. When switches S6, S7 and S8 are on, the capacitor 718 is discharged with a current substantially equal to the sum of the discharge current I_(DIS) and the extension current I_(EXT). For the example shown in FIG. 7, the magnitude of the extension current I_(EXT) partially determines the value of slope m₂ while the magnitudes of both the discharge current I_(DIS) and the extension current I_(EXT) partially determine the value of m₃. In other words, the magnitude of the extension current I_(EXT) corresponds to slope m₂ while the magnitude of discharge current I_(DIS) corresponds to the difference between slope m₃ and slope m₂ (i.e., I_(DIS)=m₃−m₂). For the example where slope m₂ is substantially one half of slope m₃, the magnitude of the discharge current I_(DIS) is substantially equal to the extension current I_(EXT). In some embodiments, the timing circuit 704 alters the amount of time it takes for the timing voltage V_(TIM) to reach the lower reference voltage V_(L) 702 by altering the speed at which the capacitor 718 discharges to the lower reference voltage V_(L) 702 and therefore altering the switching frequency f_(S) and switching period of T_(S) of the switch S1 110.

The output of comparator 706 is coupled to the inputs of latch 710. In the example shown, the output of comparator 706 is received at both the S-input of latch 710 and the R-input of latch 710. However, the small circle at the R-input of latch 710 indicates that the latch 710 receives the inverted output of comparator 706. In other words, the latch 710 will receive a logic high value at the S-input and a logic low value at the R-input, or vice versa.

Latch 710 has two outputs, the Q-output and the Q bar-output. When the latch 710 receives a logic high value at the S-input, the Q-output is logic high while the Q bar-output is logic low. When the R-input of latch 710 receives a logic high value, the Q-output is logic low while the Q bar-output is logic high. The outputs of the latch 710 are coupled to control the switching of switches S9 and S10. In the example shown, the Q bar-output controls the switching of switch S9 while the Q-output controls switching of switch S10. In one embodiment, a logic high value conesponds to a closed (i.e. ON) switch while a logic low value conesponds to an open (i.e. OFF) switch. Further, the Q bar-output of latch 710 is utilized as the clock signal 416.

Capacitor 718 further couples to comparator 726 such that the timing voltage V_(TIM) is received at the non-inverting input. Comparator 726 receives the max duty ratio voltage V_(DM) 705 at the inverting input of comparator 726. As mentioned above, the max duty ratio voltage V_(DM) 705 ensures a minimum off-time of the switch S1 110. For the example of FIG. 7, the point which the timing voltage V_(TIM) falls below the max duty ratio voltage V_(DM) 705 corresponds to when the switch S1 110 has reached the maximum duty ratio. As illustrated in FIGS. 5A, 5B and 6, the timing voltage V_(TIM) reaches the max duty ratio voltage V_(DM) 705 twice during any given switching period T_(S), however, it is the second instance of the timing voltage reaching the max duty ratio voltage V_(DM) 705 which corresponds to the maximum duty ratio. In one embodiment, the maximum duty ratio is 62%.

The AND gate 720 is coupled to receive the inverted output of comparator 726 (as shown by the small circle at the input of AND gate 720) and the Q-output of latch 710. The AND gate 720 is further coupled to the R-input of latch 732 and the on-time signal U_(ON) 418 is received at the S-input of latch 732. As such, the DCMAX signal 410 is logic high at the rising edge of the on-time signal U_(ON) 418 and transitions to a logic low value when the latch 732 after the timing voltage V_(TIM) has reached the upper reference voltage V_(H) 701 and when the timing voltage V_(TIM) has fallen below the max duty ratio voltage V_(DM) 705. Thus, the output of the AND gate 720 indicates the time at which the timing voltage V_(TIM) has fallen below the max duty ratio voltage V_(DM) 705 after reaching the upper reference voltage V_(H) 701, which is the second instance of the timing voltage reaching the max duty ratio voltage V_(DM) 705 within a given switching period. In the example shown, the length of the logic high section of the DCMAX signal 410 corresponds to the maximum duty ratio.

The output of comparator 726 is further received at AND gate 722. AND gate 722 further receives the on-time signal U_(ON) 418. The output of AND gate 722 is utilized as the extension EXT signal 736. Extension signal EXT 736 provides information regarding the extended on-time t_(ONX) as discussed with the figures above. In the example shown, the extension signal EXT 736 provides information regarding the end of the extended on time t_(ONX). The extension signal EXT 736 is received at inverter 728 which is then further coupled to control switching of switch S8. In one example, switch S8 is closed (i.e. ON) when the extension signal 736 is logic low and the switch S8 is open (i.e. OFF) when the extension signal is logic high. In operation, the AND gate 722 is logic high when the output of comparator 726 is logic high and the on-time signal U_(ON) 418 is logic high. At the beginning of the switching cycle, extension signal EXT 736 is logic low. Extension signal EXT 736 transitions to a logic high value once the timing voltage V_(TIM) 718 reaches the max duty ratio voltage V_(DM) 705. Extension signal EXT 736 then transitions to a logic low value at the end of the on-time t_(ON). The extension signal EXT 736 may also transition to a logic low value when the timing voltage V_(TIM) 718 falls below the max duty ratio voltage V_(DM) 705. Thus, the output of the AND gate 722 may be utilized to indicate the end of the extended on time t_(ONX).

In operation, the capacitor 718 charges when switch S5 is closed and switches S6 and S7 are open such that capacitor 718 is charged by current source 712 at the beginning of a switching period. The timing voltage V_(TIM) on capacitor 718 increases with a slope determined by the charging current I_(c) and the size of capacitor 718. Also, at the beginning of a switching period, switch S9 is closed and switch S10 is open. As such, the comparator 706 receives the upper reference voltage V_(H) 701 and the timing voltage V_(TIM) is compared to the upper reference voltage V_(H) 701. When the timing voltage V_(TIM) reaches the upper reference voltage V_(H) 701, as registered by the CRT signal 734 indicating that the critical time t_(C) has been reached, switch S5 turns off and switches S6 and S7 turn on. In addition, switch S9 turns off and switch S10 opens such that the comparator 706 now receives the lower reference voltage V_(L) 702.

If the on-time t_(ON) is less than the critical time t_(C), the extension signal EXT 736 is logic low for the remainder of the switching period after the critical time t_(C) is reached and as a result, switch S8 is on after the critical time t_(C) is reached. With switches S6, S7 and S8 all on, capacitor 718 is discharged with both current sink 714 and current sink 716 in the normal discharging mode. In the normal discharging mode, capacitor 718 discharges with a slope determined by discharge current I_(DIS) and the extension current I_(EXT) until the timing voltage V_(TIM) reaches the lower reference voltage V_(L) 702.

However, if the on-time t_(ON) is greater than the critical time t_(C) then the controller switches to the alternative discharging mode and the extension signal EXT 736 switches to a logic high value. In addition, provided the timing voltage V_(TIM) is still greater than the max duty ratio voltage VDM 705, indicating that the maximum duty ratio has not yet been reached, then the output of inverter 728 is also logic low. As such, the switch S8 is opened and current sink 714 is prevented from discharging the capacitor 718. Thus, in the alternative discharging mode, capacitor 718 may be discharged with current sink 716 and not current sink 714 with a slope determined by extension current I_(EXT). Switch S8 turns on again, to return the controller to the normal discharging mode, if either the on-time t_(ON) ends or the timing voltage V_(TIM) drops to the max duty ratio voltage V_(DM) 705.

Referring next to FIG. 8, a timing diagram illustrating various waveforms of voltages and currents of the timing circuit 704 of FIG. 7 is shown including a switch current ID 802, a on-time signal U_(ON) 804, a critical signal CRT 806, an extension signal EXT 808, a timing voltage VTIM 810, a clock signal CLK 812, and a DCMAX signal 814. Further illustrated in FIG. 8 are switching periods T_(S) 816, 818, and 820. Illustrated in each switching period T_(S) 816, 818, and 820 are respective on-time t_(ON), off-time t_(OFF), critical time t_(C), and base period T0. Switching periods T_(S) 818 and 820 also illustrate an extended on-time t_(ONX) and an extension period TX. The timing voltage VTIM 810 also illustrates an upper reference voltage V_(H), a max duty ratio voltage V_(DM), and a lower reference voltage V_(L). In addition, timing voltage VTIM 810 may increase to the upper reference voltage V_(H) with slope m₁. Timing voltage VTIM 810 may also decrease to the lower reference voltage V_(L) with slope m₂ or m₃.

During switching period T_(S) 816, the on-time t_(ON) is less than the critical time t_(C). At the beginning of switching period T_(S) 816, the clock signal CLK 812 pulses to a logic high value and the drive signal 804 transitions to a logic high value. The switch S1 110 turns on and the switch current ID 802 begins to increase. When the switch current ID 802 reaches the current limit I_(TIM), the switch S1 110 turns off and the on-time signal U_(ON) 804 transitions to the logic low value. At the start of the switching period, switch S9 is on and switch S10 is off (see FIG. 7) and the timing voltage VTIM 810 is compared to the upper reference voltage V_(H). The critical signal CRT 806 is the output of comparator 706. As such, the critical signal CRT 806 is logic low at the start of the switching period T_(S) 816 and transitions to the logic high value when the timing voltage VTIM 810 is substantially equal to the upper reference voltage V_(H). As mentioned above, the upper reference voltage V_(H) corresponds to the value of the timing voltage VTIM when a time period substantially equal to the critical time t_(C) has been reached. At the beginning of the switching period, the critical signal CRT 806 is logic low and switch S5 is on while switches S6 and S7 are off. The timing voltage VTIM 810 increases with slope m₁ (corresponding to when capacitor 718 is charged by current source 712 with charge current I_(C)) until the upper reference voltage V_(H) is reached. Once the timing voltage VTIM 810 reaches the upper reference voltage V_(H), switch S9 turns off and switch S10 turns on and the timing voltage VTIM 810 is compared to the lower reference voltage V_(L). In addition, the critical signal CRT 806 transitions from a logic low value to a logic high value and switch S5 turns off while switches S6 and S7 are turned on. Further, the clock signal CLK 812 transitions to a logic low value.

However, in the example of switching period 816, the on-time t_(ON) provided by the on-time signal U_(ON) 804 is less than the critical time t_(C). As a result, the extension signal EXT 808 is logic low for the remainder of the switching period T_(S) 816 after the critical time t_(C) is reached. As such, switch S8 is on at least after the critical time t_(C) is reached for the remainder of the switching period T_(S) 816. The timing voltage VTIM 810 decreases with slope m₃ (corresponding to capacitor 718 discharging through current sink 714 and 716 with the sum of the discharge current I_(DIS) and the extension current I_(EXT)) until the lower reference voltage V_(L) is reached. As shown in FIG. 8, the magnitude of slope m₃ is substantially equal to the magnitude of slope m_(E) Clock signal CLK 812 pulses to the logic high value indicating the start of switching period T_(S) 818 when the lower reference voltage V_(L) is reached.

During switching period T_(S) 818, the clock signal CLK 812 pulses to a logic high value and the drive signal 804 transitions to a logic high value signaling the beginning of the switching period. Switch S1 110 turns on and the switch current ID 802 begins to increases. As shown in FIG. 8, the switch current ID 802 has not reached the current limit I_(LIM) at the critical time t_(C). However, the switch current ID 802 reaches the current limit ILIM before the DC MAX signal 814 transitions to the logic low value.

Since the on-time t_(ON) is greater than the critical time t_(C) in switching period 818, the timing voltage VTIM 810 reaches the upper reference voltage V_(H) before the switch current ID 802 reaches the current limit I_(LIM). When the critical time t_(C) is reached, the critical signal CRT 806 transitions to the logic high value and the clock signal CLK 812 transitions to a logic low value. Since the on-time signal U_(ON) 804 is still logic high when the max duty ratio voltage VDM is reached, the extension signal EXT 808 transitions to the logic high value and switch S8 turns off. As such, timing voltage VTIM decreases with slope m₂ (corresponding to when capacitor 718 is only discharged by current sink 716 with extension current I_(EXT)).

The extension signal EXT 808 transitions to the logic low value prior to the timing voltage VTIM 810 falling below the max duty ratio voltage V_(DM). Once the extension signal EXT 808 transitions to the logic low value, the output of the inverter 728 is logic high and switch S8 is turned on for the remainder of the switching period T_(S) 818. The timing voltage VTIM 810 then decreases with slope m₃ (corresponding to capacitor 718 discharging through current sink 714 and 716 with the sum of the discharge current I_(DIS) and the extension current I_(EXT)) until the lower reference voltage V_(L) is reached. Clock signal CLK 812 pulses to the logic high value indicating the start of the next switching period T_(S) 820 when the lower reference voltage V_(L) is reached.

During switching period T_(S) 820, the on-time t_(ON) is greater than the critical time t_(C), however, the switch current ID 802 does not reach the current limit I_(LIM) before the DC MAX signal 814 transitions to the logic low value. As a result, the drive signal 128 transitions to the logic low value and switch S1 110 turns off because the maximum duty ratio was reached. At the beginning of switching period T_(S) 820, the clock signal CLK 812 pulses to a logic high value and the drive signal 128 transitions to a logic high value. Switch S1 110 turns on and the switch current ID 802 begins to increases. As shown in FIG. 8, the switch current ID 802 has not reached the current limit I_(LIM) at the critical time t_(C).

Since the on-time t_(ON) is greater than the critical time t_(C), the timing voltage VTIM 810 reaches the upper reference voltage V_(H) before the switch current ID 802 reaches the current limit I_(LIM). When the critical time t_(C) is reached, the critical signal CRT 806 transitions to the logic high value and the clock signal CLK 812 transitions to a logic low value. Since the on-time signal U_(ON) is still logic high when the max duty ratio voltage VDM is reached, the extension signal EXT 808 transitions to the logic high value and switch S8 is turned on. As such, timing voltage VTIM decreases with slope m2 (corresponding to when capacitor 718 is only discharged by current sink 716 with extension current I_(EXT)).

The extension signal EXT 808 transitions to the logic low value because the timing voltage VTIM 810 falls below the max duty ratio voltage V_(DM). Once the timing voltage VTIM 810 reaches the max duty ratio voltage V_(DM) and falls below the max duty ratio voltage V_(DM), the output of comparator 726 is logic low and the AND gate 722 receives the logic low value. As such, the switch S8 is turned on for the remainder of switching period T_(S) 820. The timing voltage VTIM 810 decreases with slope m₃ (corresponding to capacitor 718 discharging through current sink 714 and 716 with the sum of the discharge current I_(DIS) and the extension current I_(EXT)) until the lower reference voltage V_(L) is reached.

In addition, the AND gate 720 receives logic high values from both the Q-output of latch 710 and the inverted output of comparator 726. Latch 732 resets and the DCMAX signal 814 transitions to the logic low value. The drive signal 128 transitions to the logic low value in response to the DCMAX signal 814 and the switch S1 110 turns off (corresponding to the end of the on-time t_(ON) of the switch current ID 802 in switching period T_(S) 820).

While the invention herein disclosed has been described by means of specific embodiments, examples and applications thereof, numerous modifications and variations could be made thereto by those skilled in the art without departing from the scope of the invention set forth in the claims. 

What is claimed is:
 1. An integrated circuit controller for use in a switching power supply, the controller comprising: a pulse width modulation (PWM) circuit to be coupled to control a switch to regulate an output of the power supply in response to a switch current flowing through the switch and in response to a clock signal having a switching period; and a timing circuit coupled to the PWM circuit to provide the clock signal, wherein the timing circuit increases the switching period of the clock signal in response to an on time of the switch exceeding a threshold time, wherein the switching period of the clock signal is a fixed switching period if the on time of the switch is less than the threshold time and wherein the timing circuit increases the switching period to greater than the fixed switching period if the on time is greater than or equal to the threshold time.
 2. The integrated circuit controller of claim 1, wherein the threshold time is equal to one-half the fixed switching period.
 3. The integrated circuit controller of claim 1, wherein the timing circuit increases the switching period by an amount of time that is responsive to a difference in time between the on time of the switch and the threshold time.
 4. The integrated circuit controller of claim 3, wherein the amount of time that the timing circuit increases the switching period is proportional to the difference in time between the on time of the switch and the threshold time.
 5. The integrated circuit controller of claim 1, wherein the PWM circuit is coupled to turn off the switch in response to the switch current reaching a current limit.
 6. The integrated circuit controller of claim 5, wherein the PWM circuit is coupled to receive a feedback signal representative of the output of the power supply and wherein the current limit is a variable current limit responsive to the feedback signal.
 7. The integrated circuit controller of claim 1, wherein the switch is included in the integrated circuit controller.
 8. The integrated circuit controller of claim 1, wherein the timing circuit includes a timing capacitor, and wherein the switching period of the clock signal is equal to a charging time that the timing capacitor charges to an upper reference voltage plus a discharging time that the timing capacitor discharges to a lower reference voltage.
 9. The integrated circuit controller of claim 8, wherein the timing circuit increases the discharging time of the timing capacitor by decreasing a rate at which the timing capacitor is discharged to extend the switching period if the on time is greater than or equal to the threshold time.
 10. The integrated circuit controller of claim 8, wherein the timing circuit discharges the timing capacitor at a first rate when the on time of the switch is less than the threshold time and wherein the timing circuit discharges the timing capacitor at a second rate when the on time of the switch is greater than or equal to the threshold time, wherein the first rate is greater than the second rate.
 11. The integrated circuit controller of claim 10, wherein the timing circuit discharges the timing capacitor at the second rate for a remainder of the on time that begins when the threshold time is reached.
 12. The integrated circuit controller of claim 10, wherein the timing circuit discharges the timing capacitor at the second rate only until the switch turns off and then resumes discharging the timing capacitor at the first rate to the lower reference voltage.
 13. The integrated circuit controller of claim 10, wherein the second rate is less than or equal to one-half the first rate.
 14. The integrated circuit controller of claim 10, wherein a ratio of second rate to the first rate varies inversely proportional to a duty ratio of the switch during the switching period.
 15. The integrated circuit controller of claim 10, wherein the timing circuit further comprises: a first current source coupled to charge the timing capacitor to the upper reference voltage; and a second and a third current sink coupled to discharge the timing capacitor to the lower reference voltage, wherein the second current sink and the third current sink are enabled together to discharge the timing capacitor at the first rate when the on time of the switch is less than the threshold time, and wherein the second current sink is disabled and the third current sink enabled to discharge the timing capacitor at the second rate when the on time of the switch is greater than or equal to the threshold time.
 16. The integrated circuit controller of claim 8, wherein the timing circuit discharges the timing capacitor at a first rate when the on time of the switch is less than the threshold time and wherein the timing circuit maintains a constant voltage on the timing capacitor until the switch turns off and then discharges the timing capacitor at the first rate if the on time of the switch is greater than or equal to the threshold time.
 17. A switching power supply, comprising: an energy transfer element coupled to transfer energy between and input and an output of the switching power supply; a switch coupled to control the transfer of energy through the energy transfer element; and a controller coupled to provide a drive signal to control the switch to regulate the output of the switching power supply, wherein the controller includes: a pulse width modulation (PWM) circuit coupled to generate the drive signal in response to a switch current flowing through the switch and in response to a clock signal having a switching period; and a timing circuit coupled to the PWM circuit to provide the clock signal, wherein the timing circuit increases the switching period of the clock signal in response to an on time of the switch exceeding a threshold time, wherein the switching period of the clock signal is a fixed switching period when the on time of the switch is less than the threshold time and wherein the timing circuit increases the switching period to greater than the fixed switching period if the on time is greater than or equal to the threshold time.
 18. The switching power supply of claim 17, wherein the threshold time is equal to one-half the fixed switching period.
 19. The switching power supply of claim 17, wherein the timing circuit increases the switching period by an amount of time that is responsive to a difference in time between the on time of the switch and the threshold time.
 20. The switching power supply of claim 19, wherein the amount of time that the timing circuit increases the switching period is proportional to the difference in time between the on time of the switch and the threshold time.
 21. The switching power supply of claim 17, wherein the PWM circuit is coupled to turn off the switch in response to the switch current reaching a current limit.
 22. The switching power supply of claim 17, wherein the PWM circuit is coupled to receive a feedback signal representative of the output of the power supply and wherein the current limit is a variable current limit responsive to the feedback signal.
 23. The switching power supply of claim 17, wherein the switch and the controller are included in an integrated circuit.
 24. A method, comprising: generating a clock signal having a switching period; increasing the switching period of the clock signal in response to an on time of a switch included in a power supply exceeding a threshold time, wherein the switching period of the clock signal is a fixed switching period when the on time of the switch is less than the threshold time; and controlling the switch to regulate an output of the power supply in response to a switch current flowing through the switch and in response to the clock signal.
 25. The method of claim 24, wherein an amount of time that the switching period is increased is proportional to the difference in time between the on time of the switch and the threshold time.
 26. The method of claim 24, further comprising turning off the switch in response to the switch current reaching a current limit. 